To evaluate the importance of the immune system in the pathogenesis of pulmonary fibrosis, the susceptibility of the nude, athymic mouse to fibrosis was tested by the administration of blemycin, a drug wich produces a high incidence of interstitial pneumonitis and fibrosis. Bleomycin administration resulted in pulmonary lesions of widely varying severity in both normal and nude mice. Both developed pulmonary fibrosis and pneumonitis; there were no differences in the susceptibility of the control mice and nude mice to bleomycin. To the extent that the athymic mice and the amounts of bleomycin used in this study are valid representations of pulmonary fibrosis in man, the importance of the cellular immune system in the pathogenesis of the fibrotic lesion has to remain questionable.